<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=, initial-scale=1.0">
    <title>demo_class_()</title>
    <style>
        .color{
            color: red;
        }
        .size{
            font-size: 66px;
        }
    </style>
</head>
<body>
    <!-- class：定义 和 使用才能生效 -->
    <p>demo_Hello-World</p>
    <P class="color size">demo_Hello-World(P class=".use")</P>
    <div class="color">demo_class()</div>
</body>
</html>